Inkjet printing systems include a printhead having an array of drop ejectors that are controlled to eject ink in an imagewise fashion on a printing medium. The quality of the image is determined by factors including tone density uniformity and color rendition that depend somewhat on the volume of the drops of ink that are ejected. If there is excessive variability of the drop volume from one printed image to another, the appearance differences between the images may be objectionable.
It is well known that there are a variety of factors that can influence drop volume. These include drop ejector design, manufacturing variability, physical properties of the ink, temperature of the printhead and ink, pulse waveform for actuating the drop ejector, and drop ejector aging effects. Once a printhead has been designed and an ink has been chosen, the nominal drop volume is determined and the goal becomes one of keeping drop volume variation acceptably low during operation. Generally, drop volume increases with the temperature of the ink, and the modification of the drop ejection actuation waveform or pulse parameters as a function of temperature in order to maintain drop volume approximately constant has been disclosed, for example, in U.S. Pat. No. 5,036,337.
However, there are still other sources of variation in drop volume. Two of these are related to ink supply. As disclosed in U.S. Pat. No. 6,517,175, the drop volume can also be dependent on how much ink remains in the ink reservoir that supplies ink to the printhead, as well as on the ink flow rate for printing that depends on the pattern to be printed. For example, for an ink supply tank containing a porous capillary medium that supplies a negative pressure to the printhead so that ink does not leak out the drop ejector nozzles, a greater negative pressure is provided by the capillary medium when the ink supply tank contains less ink. As a result, the ink meniscus at the nozzles is more concave, so that the ejected drop volume is smaller when there is less ink remaining in the ink tank.
What is needed is a method of printing that compensates for variations in the ink supply, in order to provide a more nearly constant drop volume.